The present invention relates to a method for forming a multi-color toner image. More particularly, the present invention relates to a method for forming a multi-color toner image in which a multi-color toner image is formed by developing at least two electrostatically latent images formed on one photosensitive material by toners differing in the color.
Development of a printer capable of recording a toner image having at least two colors as a terminal device of a computer or the like has been desired. A printer according to the electrophotographic process is practical as the printer of this type, and a laser beam printer has been proposed. In a two-color laser printer, a rotary photoconductive photosensitive drum is uniformly charged and the photosensitive drum is subjected to negative light exposure with a first laser beam modulated by a first image information to form a first electrostatic latent image, and this first electrostatic latent image is subjected to reversal development in a first development apparatus to cause a first toner to adhere to a charge-released region and form a first toner image. Then, the photosensitive drum is subjected again to negative light exposure with a second laser beam modulated by a second image information to form a second electrostatic latent image, and this second electrostatic latent image is subjected to reversal development in a second development apparatus to cause a second toner to adhere to the charge-discharged region and form a second toner image. The two-color toner image thus formed on the surface of the photosensitive drum is transferred onto a recording paper and fixed thereto.
In the method for forming a two-color toner image by the above-mentioned laser beam printer, while the second electrostatic latent image is developed, a part of the first toner which has already adhered to the photosensitive drum is separated from the drum and incorporated in the second development apparatus, and with increase of the amount of the incorporated first toner, the second toner image becomes turbid.
As the method for normally developing two colors, there has been proposed in Japanese laying-open of patent application No. 55-87173 a method in which when the other of the first development apparatus is incorporated in the second development apparatus, the incorporated first toner is reversely charged to the same polarity as that of the charge of the second latent image by frictional charging with a carrier of the second developer, whereby adhesion of the first toner to the photosensitive drum is inhibited and mingling of colors is thus prevented. In this method for normally developing two colors, since the second toner adheres to the periphery of the carrier of the second developer, the chance of frictional contact of the first toner with the carrier in the second developer is reduced and a long time is required for reversal of the charge polarity. Accordingly, there is a risk of adhesion of the first toner to the second electrostatic latent image before reversal of the charge polarity. Furthermore, since the toner having the charge polarity thereof reversed is left in the second development apparatus, in a system provided with for uniformly controlling the toner mixing ratio in the developer or the amount of the developer, the amount of the second toner is reduced in correspondence to the amount incorporated of the first toner, and therefore, the development capacity of the second development apparatus is reduced.